Process of coating cathode-ray tube screens



y 1948- R. B. WINDSOR 2,441,217

PROCESS OF COATING CATHODE RAY TUBE SCREENS Filed July 9, 1945 2Sheets-Sheet 1 F|G.l I3

5 Ill 0! [LI & Ill a0 5 Ill l h i I II: u 20 i 4 l E x I I o I n: l0 I iI 0 1 l I I 1 -200 -|00 0 +35 200 300 400 500 TEMPERATURE IN C INVENTQRROBERT BEACH WINDSOR ATTORNEY May 11,

1948. R. B. WINDSOR 2,441,217

PROCESS OF COATING CATHODE RAY TUBE SCREENS Filed July 9, 1945 2Sheets-Sheet 2 FIG.3

conoauseo AT 35c p CONDENSED AT -50C z g? l6 m E 53' l2- a. If '6 g so lI I MAXIMUM CONTRAST OF 36OTH. PUL$E(PERCENT) 20 FIG. 4

CONDENSED AT C CONDENSED AT-C 'z' 8 IS 5 I 4- 2 IZ- 0. DJ m a 2 IO 5 8-56 8 si- 5 I 6 A o 2 z z I TE SE I 2 4 6 8 IO l2 l4 l6 I8 20 22 24 26 283O 32 MAXlMUM CONTRAST OF IST. PULSE (PERCENT) ATTORNEY Patented May 11,1948 -ur OPFFICE' PROCESS F COATING CATHODE-RAY TUBE SCREENS RobertBeach Windsor, Cambridge, Mass, assigner, by mesne assignments, to theUnited States of America as representedbythe Secretary of WarApplication July 9, 1945, Serial No. 604,044

2 Claims.

This invention relates in general to cathoderay tube faceplate coatingprocesses, and more particularly to such processes as are applied to socalled Skiatrons, or dark trace cathode ray tubes.

The Skiatron is a color trace or dark trace cathode ray tube in which animage appears upon its screen by virtue of the areas of light absorptionwhich are set up by electron bombardment, rather than areas offluorescence as in conventional cathode ray tubes. The screen of aSkiatron normally consists of a coating of a film of a microcrystallinealkali halide, with potassium chloride being one of the substances whichis used extensively. A television projection tube making use of thepeculiar properties of microcrystalline alkali halides undersimultaneous electron excitation and illumination by light is describedin an article in the Proceedings of the Institute of Radio Engineers,volume 28, page 203 (1940) entitled A system of large-screen televisionreception based on certain electron phenomena in crystals, by A. H.Rosenthal.

One of the characteristics of the alkali halide screen used in theSkiatron tube is that a-narea excited by electron bombardment willpartially absorb any light to which it is exposed. Thus external lightwill cause such an area to show up colored in a manner visible in normalroom illumination.

Previous fSkia-tron screens were unsatisfactory because of the longpersistence of the coating to repeated excitation. This is undesirablebecause detection of new signals after the 'bom-' bardment has stoppedmust wait for the old si nals to die out to some extent. The persistenceof the screen for a single signal was satisfactory for some purposes inprevious Skiatron tubes,-

one embodiment .of the screen allowing a single signal to be'visible for5 to seconds.

Among the objects of this invention, therefore, are:

1. To provide a Skiatron screen in which the coating has a relativelylong persistence to single signal excitation.

2. To provide a Skiatron screen in which the coating has a relativelyshort persistence to repeated excitation.

3. To provide a process for applyin a. coating.

such as used-in the Skiatron.

In accordance with the present invention, there isprovided a Skiatrontubewhose facelhaslbeen cooled to from 0 C. to -90 C. by a contiguousthat noted when the screen was condensed at room temperatures. Severalcups'use'd to evaporate the alkali halide are placed at an optimum anglewith respect to the screen and the process of condensation ofthecoatingis made to last the most favorable amount of time. Theangle ofvcondensation process;

Fig. 2 is a graph of -t'he'approximate iight reflectance froma'screenoffixed-thickness which has had its' coating process applied atvarious condensationtemperatures;

Fig. 3 is a graphof the decay of the screen darkness for repeatedexcitations in screens con;-

densed' at .5'0 C(and at +35 0.; and

Fig. 4 is a graph of the decay .of the screen darkness fora singleexcitation in screens condensedat 50 andat +35 C.

Referring now'to a'description of the process, it was found afterresearch on many variables that the condensing under controlledconditions of potassium chloride on a Skiatron faceplate after it hasbeencooled to .a temperature between 0C. and .90 C. :is one desirableembodiment. Fig. 1 shows one arrangement that may be used in thecondensation-process. The Skiatron tube in has its race all contiguousto a copper block-l2. This copp r block 12 is cupped to-fit thecurvature of the face of the tube. A cup 13 has small amounts of liquidair added to it, and by this means the temperature may be :held constantwithin i2 C, A--thermocou-ple-in contact with the faceplate of the tubewill indicate when the temperature has become stabilized.- evaporatorcup t l holdingthe potassium chloride is shown in position; The cup maybe held in place-by supportsleading from the Skiat-ron high voltageelectrode lead (not shown) In order that the screen have comparative ypeated excitations the temperature of the screen.

must be .betWeenO" -.C. and C. while condensationistaking place. Thereflectance of the screen, however, which is a measure of the amount ofsignal lightereflected from it, varies in this temperature region as isshownby the graph .of Fig. .2. This graph gi-ves reflectancegin percent9f a given'value .for an angle betweenta normal tothe screen and-thedirection ofeaim of the evaporator.rcup;of;25. In oneisatisfactoryembodiment :of the 'process the screen was Z-hel'd at .a temperature ofT5U:C. 'duringthe condensa tion process.

The

Fig. 3 shows the variation in decay of the 360th pulse applied after aquiescent non-excitation conditionof the screen, with 10 secondintervals between pulses, as a function of maximum contrast, or maximumcoloration, of the 360th pulse for screens condensed at 35 C. and at -50C. The curves are labeled as to the time after the 360th pulse maximumthat the data was taken, and the decay is measured as a percent of agiven value. The contrast of the darkening, or coloration, of the screenis defined as (Lo-L) [L0, where L is the light reflected from anundarkened, or uncolored, area, and L is the light reflected from thesame area after being darkened. It is seen from this graph that thecontrast for repeated excitation of the screen after a given time fromthe maximum point of the pulse, say 1000 seconds later, is less forscreens condensed at -50 C. than for screens condensed at +35 C. Thegraph of Fig. 3 is for a Skiatron operating temperature of 35 C.

Fig. 4 shows the variation in decay of the first pulse applied after aquiescent non-excitation condition of the screen, as a function ofmaximum contrast of the first pulse, for screens condensed at 35 C. andat -50 C. The curves are labeled as to the time after the pulse maximumthat the data was taken, and the decay is measured as a percent of agiven value. It is seen from the graph. that the persistence for singlesignals, which is satisfactory in screens condensed at 35 0., is notappreciably altered in screens condensed at 50 C. This is shown by thecloseness of the two curves for say 100 seconds after the pulse maximumfor screens condensed at +35 C. and those condenser at 50 C. The graphof Fig. 3 is for an operating temperature of 35 C.

In judging the amount of alkali halide to be evaporated on the screen, acompromise must be made between reflectance, which is better the thickerthe screen, and volume scattering of the light incident upon the screen,which increases with thickness of the screen. If the thickness of thescreen is increased beyond a certain point, an undesirable number ofcomparatively large crystals of halidewill form between the face of thetube and the comparatively thin layer of halide crystals which areexcited by the electron bombardment. Hence some of the light impingingon the face of the tube will be scattered by these large crystals, andwill be reflected without containing the intelligence imparted to thescreen by the electron bombardment.

A type of evaporator cup which has been found to be most satisfactoryfor a uniform screen is one which may have its evaporation product aimedto a certain extent. For screens condensed at low temperatures, thereflectance increases with the angle between the vapor stream and anormal to the face of the tube. One desirable embodiment of the processentails the use of two evaporator cups aimed so that the minimumreflectance area given by the stream from one of the cups may be coveredby the maximum reflectance area from the stream of the other. The anglesof aim of the two cups, in this instance, were 90 apart, and 1200 mg. ofpotassium chloride was used in each cup.

The face of the tube must be kept free from impurities during thecondensation process, because potassium chloride tends to form largecrystals around small particles, with the result being an undesirableamount of light scattering. If a large amount of persistence is desired,the

4 Skiatron must be operated with the screen at as low a temperature aspracticable. Higher operating temperatures increase the. rate of decayof signals from both single and repeated excitations of all Skiatronscreens. In order to obtain a desired contrast after a given time whenhigh operational temperatures must be used, it is V necessary toincrease the initial contrast, usually done by sending a higher currentto the screen. The result is that the pattern becomes less clear due tothe larger spot diameter.

A summing up of one combination of conditions for screen condensationfor a four-inch tube which give desirable results is as follows:

1. Faceplate held at C.

2. Two evaporator cups used with 1200 mg. of potassium chloride in each,and placed about 90 apart.

3. Each cup aimed at the area of the faceplate directly above the other.

4. Pressure in the tube just prior to condensation less than 10- mm. ofmercury. Condensation period for each evaporator cup made to last atleast 10 minutes.

While there has been described what is at present considered thepreferred embodiment of th invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

I claim:

1. In the manufacture of cathode ray tubes, the process of producing areproducing screen within the tube, which process comprises cooling, toa temperature substantially between 0 and 90 0., the screen portion ofthe tube, and condensing thereon the vapor of' an alkali halide whilethe screen portion of the tube is held within said temperature rangeduring the entire condensation step whereby the persistence of thescreen, under repeated signals, is diminished, and the persistence undersingle signals is substantially unchanged.

2. In the manufacture of cathode ray tubes,

'the process of producing a reproducing screen within the tube, whichprocess comprises cooling,

to a temperature substantially between 0 and C., the screen portion ofthe tube, and condensing thereon the vapor of potassium chloride whilethe screen portion of the tube is held within said temperature rangeduring the entire condensation step whereby the persistence of thescreen, under repeated signals, is diminished, and the persistence undersingle signals is substantially unchanged.

ROBERT BEACH WINDSOR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number

